Method for a user terminal to random access a carrier aggregation mobile communication system

ABSTRACT

A method for random access to a base station by a user equipment (UE) in a mobile communication system using carrier aggregation in which communication is conducted over a plurality of component carriers, and a terminal for the method are disclosed. When the UE performs a contention-based random access procedure in the mobile communication system to which CA technology is applied, the UE considers an uplink grant signal as a contention resolution message when receiving the uplink grant signal from a eNB only when the uplink grant signal is received through a downlink CC corresponding to an uplink CC used to transmit a random access preamble or a third message, to thereby prevent contention resolution from being erroneously ended.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371application of International Application No. PCT/KR2010/003629, filed onJun. 7, 2010, which claims the benefit of U.S. Provisional ApplicationSer. Nos. 61/219,366, filed on Jun. 22, 2009 and 61/184,833, filed onJun. 7, 2009, the contents of which are incorporated by reference hereinin their entirety.

TECHNICAL FIELD

The following description relates to a method for random access to abase station by a user equipment (UE) in a mobile communication systemusing carrier aggregation in which communication is conducted over aplurality of component carriers, and a terminal for the method.

BACKGROUND ART

A brief Long Term Evolution (LTE) is a mobile communication systemevolved from Universal Mobile Telecommunications System (UMTS),standardized by an international standardization organization, the3^(rd) Generation Partnership Project (3GPP). The configuration of anLTE system is illustrated in FIG. 1.

FIG. 1 is a view referred to for describing the configuration of an LTEsystem.

The LTE system may be divided largely into an Evolved UMTS TerrestrialRadio Access Network (E-UTRA) and an Evolved Packet Core (EPC). TheE-UTRAN includes UEs and evolved Node Bs (eNBs). A UE is connected to aneNB via a Uu interface and one eNB is connected to another eNB via an X2interface. The EPC includes a Mobility Management Entity (MME)responsible for control-plane functions and a Serving GateWay (S-GW)responsible for user-plane functions. An eNB is connected to the MME viaan S1-MME interface and an eNB is connected to the S-GW via an S1-Uinterface. These two interfaces are collectively called an S1 interface.

For the Uu interface being an air interface, a radio interface protocolis defined. The radio interface protocol horizontally includes aphysical layer, a data link layer, and a network layer and verticallyincludes a user-plane (U-plane) for user data transmission and acontrol-plane (C-plane) for control signaling. Based on the lowest threelayers of the Open System Interconnection (OSI) reference model, thisradio interface protocol can be divided into Layer 1 (L1) including aphysical layer PHY, Layer 2 (L2) including a Medium Access Control/RadioLink Control/Packet Data Convergence Protocol (MAC/RLC/PDCP) layer, andLayer 3 (L3) including a Radio Resource Control (RRC) layer. Theselayers are defined in pairs between a UE and an E-UTRAN, for datatransmission via the Uu interface.

Now a description will be given below of a Long-Term Evolution Advanced(LTE-A) system.

LTE-A is a system developed from LTE to meet 4^(th) Generation mobilecommunication requirements, that is, IMT-Advanced requirementsrecommended by the International Telecommunication Union-Radiocommunication sector (ITU-R). The 3GPP which developed the LTE systemstandard is now actively working on standardization of the LTE-A system.

Major technologies added to the LTE-A system are carrier aggregation forextending a used bandwidth and flexibly using the bandwidth and use ofrelays for improving coverage, supporting group mobility and enablinguser-centered network deployment.

FIGS. 2 and 3 are views referred to for describing radio protocollayers.

At L1, the PHY layer provides information transfer service to a higherlayer on physical channels. The PHY layer is connected to the MAC layerthrough transport channels and data is transferred between the MAC layerand the PHY layer on the transport channels. The transport channels arelargely divided into dedicated transport channels and common transportchannels depending on whether the transport channels are shared or not.Data is transmitted on physical channels using radio resources betweendifferent PHY layers, that is, the PHY layers of a transmitter and areceiver.

There is a plurality of layers at L2. The MAC layer maps logicalchannels to transport channels and performs logical channel multiplexingby mapping a plurality of logical channels to one transport channel. TheMAC layer is connected to a higher layer, namely the RLC layer, throughlogical channels. Depending on the types of information carried on thelogical channels, the logical channels are classified into controlchannels that deliver C-plane information and traffic channels thatdeliver U-plane information.

The RLC layer at L2 adjusts a data size to be suitable for datatransmission in the air interface from a lower layer by segmenting andconcatenating data received from a higher layer. In order to guaranteevarious Quality of Service (QoS) requirements of each Radio Bearer (RB),the RLC layer provides three operation modes, Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). Especially, an AMRLC performs a retransmission function through Automatic Repeat andRequest (ARQ), for reliable data transmission.

The PDCP layer at L2 compresses a header to reduce the size of anInternet Protocol (IP) packet header of a relatively large sizecontaining unnecessary control information to efficiently transmit an IPpacket such as an IPv4 or IPv6 packet via a radio link having a narrowbandwidth. The header compression function enables transmission ofnecessary information in a header only, thereby increasing thetransmission efficiency of a radio link. In addition, the PDCH layerperforms a security function in the LTE system. This security functioninvolves ciphering for preventing a third party from eavesdropping andintegrity protection for preventing a third party from maliciouslymodifying data.

The RRC layer at the highest of L3 is defined only in the C-plane. TheRRC layer takes charge of controlling logical channels, transportchannels, and physical channels in relation to configuration,reconfiguration, and release of Radio Bearers (RBs). An RB is a logicalpath provided by L1 and L2 in the radio protocol architecture, for datatransmission between a UE and a UTRAN. In general, configuring an RBmeans defining the features of a radio protocol layer and channelsneeded to provide a specific service and setting specific parameters andan operation scheme. RBs are classified into a Signaling RB (SRB) andData RB (DRB). The SRB is used as a path via which an RRC message istransmitted on the C-plane and the DRB is used as a path in which userdata is transmitted on the U-plane.

Now a description will be given of Carrier Aggregation (CA) in the LTE-Asystem.

FIG. 4 is a view referred to for describing CA.

As described above, the LTE-A standard is designed as an IMT-Advancedcandidate technology of the ITU to satisfy IMT-Advanced technicalrequirements. Accordingly, extension of a bandwidth from the legacy LTEsystem is under discussion to satisfy IMT-Advanced technicalrequirements. For bandwidth extension, carriers available to the legacyLTE system are defined as Component Carriers (CCs) in the LTE-A system.Aggregation of up to 5 CCs is under discussion, as illustrated in FIG.4. Because a CC may occupy up to 20 MHz as in the LTE system, the CAtechnology of the LTE-A standard is a concept of extending a bandwidthto up to 100 MHz. The technology of aggregating a plurality of CCs iscalled CA.

Hereinbelow, a random access procedure performed in the LTE system willbe described in greater detail.

In the LTE system, a UE may perform the random access procedure,

when the UE initially accesses an eNB without an RRC connection havingbeen established therebetween,

when the UE initially accesses a target cell during handover,

when the random access procedure is requested by a command from the eNB,

upon generation of uplink data in a situation in which uplink timesynchronization has not been acquired or specified radio resources foruse in requesting radio resources have not been allocated, or

when radio link failure or handover failure is recovered.

Based on the above description, a general contention-based random accessprocedure will be described below.

FIG. 5 is a diagram illustrating a signal flow for operations of a UEand an eNB in a contention-based random access procedure.

(1) Transmission of First Message

The UE may select a random access preamble randomly from a randompreamble set indicated by system information or a handover command,select Physical Random Access CHannel (PRACH) resources, and transmitthe random access preamble through selected Physical PRACH (PRACH)resources (S501).

(2) Reception of Second Message

After transmitting the random access preamble in step S501, the UEattempts to receive a random access response within a random accessresponse reception window indicated through the system information orthe handover command by the eNB (S502). To be more specific, the randomaccess response may be transmitted in the form of a Medium AccessControl Protocol Data Unit (MAC PDU) and the MAC PDU may be deliveredover a Physical Downlink Shared CHannel (PDSCH). To receive informationon the PDSCH successfully, the UE preferably monitors a PhysicalDownlink Control CHannel (PDCCH). That is, the PDCCH preferably carriesinformation about a UE to receive the PDSCH, information about thefrequency and time of radio resources of the PDSCH, and informationabout the transmission format of the PDSCH. Once the UE succeeds inreceiving the PDCCH destined therefor, the UE may successfully receive arandom access response over the PDSCH according to information carriedover the PDCCH. The random access response may include an identifier(ID) of the random access preamble (e.g. a Random Access Preamble ID(RAPID)), an Uplink (UL) Grant indicating uplink radio resources, atemporary Cell-Radio Network Temporary Identify (C-RNTI), and a TimingAdvance Command (TAC).

The reason for including the RAPID in the random access response is thatbecause one random access response may contain random access responseinformation for one or more UEs, it is necessary to indicate a UE towhich the UL Grant, the temporary C-RNTI, and the TAC are valid. It isassumed in step S502 that the ID of the random access preamble isidentical to the RAPID included in the random access response. Thus, theUE may receive the UL Grant, the temporary C-RNTI, and the TAO.

(3) Transmission of Third Message

Upon receipt of a valid random access response, the UE processesinformation included in the random access response. That is, the UEapplies the TAO and stores the temporary C-RNTI. In addition, the UE maystore data to be transmitted in a message3 buffer in correspondence withthe reception of a valid random access response.

Meanwhile, the UE transmits data (i.e. a third message) to the eNB usingthe received UL Grant (S503). The third message should include an ID ofthe UE. In the contention-based random access procedure, the eNB cannotidentify UEs that perform the random access procedure. However, the eNBshould identify the UEs to avoid later-collision among them.

Two methods have been discussed to include the ID of the UE in the thirdmessage. One of the methods is that if the UE has a valid C-RNTIallocated by the cell before the random access procedure, the UEtransmits its C-RNTI in an uplink signal corresponding to the UL Grant.On the other hand, if a valid C-RNTI has not been allocated to the UEbefore the random access procedure, the UE transmits its UE ID (e.g.S-TMSI or a random ID) in data. In general, the UE ID is longer than theC-RNTI. If the UE transmits data corresponding to the UL Grant, the UEactivates a Contention Resolution (CR) timer to avoid contention.

(4) Reception of Fourth Message

After transmitting its ID in data according to the UL Grant included inthe random access response, the UE awaits reception of a command forcontention resolution from the eNB. That is, the UE attempts to receivea PDCCH in order to receive a specific message (S504). For PDCCHreception, two methods may be considered. When the third message istransmitted using the C-RNTI according to the UL Grant as describedabove, the UE attempts to receive a PDCCH using the C-RNTI. If the IDincluded in the third message is the UE ID, the UE may attempt toreceive a PDCCH using the temporary C-RNTI included in the random accesspreamble. In the former case, if the UE receives a PDCCH using theC-RNTI before expiration of the CR timer, the UE ends the random accessprocedure, determining that the random access procedure has beenperformed normally. In the latter case, if the UE receives a PDCCH usingthe temporary C-RNTI before expiration of the CR timer, the UE checksdata received on a PDSCH indicated by the PDCCH. If the data includesits UE ID, the UE ends the random access procedure, determining that therandom access procedure has been performed normally.

Meanwhile, a contention free random access procedure is ended only bytransmitting first and second messages, which is different from thecontention-based random access procedure shown in FIG. 5. However, theUE is allocated a random access preamble by the eNB before it transmitsa random access preamble as the first message to the eNB. The UEtransmits the allocated random access preamble as the first message tothe eNB, and ends the random access procedure by receiving a randomaccess response from the eNB.

DISCLOSURE Technical Problem

When the UE attempts to perform contention-based random access to theeNB in the LTE system, a contention resolution procedure may beerroneously ended due to confusion between an uplink grant fortransmission of a corresponding Buffer Status Report (BSR) and uplinkgrant for transmission of another Buffer Status Report (BSR). Althoughthis problem is an extremely unusual case, if the above-described CAtechnology is applied to the LTE-A system, the probability of generationof erroneous completion of contention resolution due to confusionbetween uplink grant information about a corresponding CC and uplinkgrant information about other CCs is increased.

Accordingly, a description will be given of a method for performingrandom access at a UE without the above-mentioned confusion in a mobilecommunication system to which the CA is applied, and a UE configured toperform the method.

Technical Solution

The object of the present invention can be achieved by providing amethod for performing a random access procedure by a User Equipment (UE)for an eNB in a mobile communication system using carrier aggregation inwhich communication is conducted using a plurality of componentcarriers, the method including: transmitting a random access preamble tothe eNB through a first uplink component carrier (hereinafter referredto as ‘UL CC’) from among a plurality of UL CCs; receiving a randomaccess response message including first uplink (UL) grant information asa response to the random access preamble through a first downlinkcomponent carrier (hereinafter referred to as ‘DL CC’) corresponding tothe first UL CC; transmitting a third message including identityinformation of the UE to the eNB through an uplink radio resourcecorresponding to the first UL grant information in the first UL CC; andreceiving second UL grant information from the eNB, wherein the UEconsiders the contention resolution procedure according to transmissionof the third message as a successful procedure only when the second ULgrant information is received through the first DL CC.

The UE may not consider the second UL grant information PDCCH as thecontention resolution message according to transmission of the thirdmessage when the second UL grant information is received through asecond DL CC different from the first DL CC.

The UE may include a physical layer module and a Medium Access Control(MAC) module, and the MAC layer module may determine whether thecontention resolution procedure according to transmission of the thirdmessage is successful or not only when the physical layer module reportsreception of a Physical Downlink Control Channel (PDCCH) through thefirst DL CC to the MAC layer module.

The identity information of the UE may be a C-RNTI MAC control elementwhich indicates C-RNTI of the UE. When PDCCH including the second ULgrant information is received through the first DL CC and indicates theC-RNTI of the UE, the UE may consider the contention resolutionprocedure according to transmission of the third message as a successfulprocedure.

The identity information of the UE may be a UE contention resolutionidentity MAC control element which includes an identifier other than theC-RNTI of the UE. When the PDCCH including the second UL grantinformation is received through the first DL CC and indicates temporaryC-RNTI of the UE and a PDSCH corresponding to the PDCCH includes the UEcontention resolution MAC control element, the UE may consider thecontention resolution procedure according to transmission of the thirdmessage as a successful procedure.

The UE may consider whether the PDCCH is received through the first DLCC or not in order to determine whether the contention resolutionprocedure according to transmission of the third message is successfulonly when the UE transmits the third message including the C-RNTI MACcontrol element to the eNB.

The UE may perform a HARQ operation through a second UL CC differentfrom the first UL CC and a second DL CC different from the first DL CC,independently of transmission of the third message and reception of thesecond UL grant information.

In another aspect of the present invention, provided herein is a UEwhich performs random access to an eNB in a mobile communication systemusing carrier aggregation in which communication is conducted using aplurality of component carriers, the UE including: a MAC layer moduleincluding a plurality of HARQ entities respectively corresponding to theplurality of component carriers, and configured to control signaltransmission using a plurality of uplink component carriers (referred toas ‘UL CCs’) through the plurality of HARQ entities and signal receptionthrough a plurality of downlink component carriers (referred to as ‘DLCCs’) respectively corresponding to the plurality of UL CCs; and aprocessor functionally connected to the MAC layer module and including aphysical layer module configured to transmit a signal through theplurality of UL CCs and to receive a signal through the plurality of DLCCs, wherein only when UL grant information is received through a firstDL CC corresponding to a first UL CC used to transmit a third message tothe eNB from among the plurality of UL CC, the processor considers acontention resolution procedure according to transmission of the thirdmessage as a successful procedure.

The processor may be configured such that the processor does notconsider UL grant information received through a second DL CC differentfrom the first DL CC as the contention resolution message according totransmission of the third message.

The MAC layer module may be configured to determine whether thecontention resolution procedure according to transmission of the thirdmessage is successful or not only when the physical layer module reportsreception of a PDCCH through the first DL CC to the MAC layer module.

The third message may be identity information of the UE and include aC-RNTI MAC control element which indicates C-RNTI of the UE, and theprocessor may consider the contention resolution procedure according totransmission of the third message as a successful procedure when PDCCHincluding the UL grant information is received through the first DL CCand indicates the C-RNTI of the UE.

The third message may be identity information of the UE and include a UEcontention resolution identity MAC control element which includes anidentifier other than C-RNTI of the UE, and the processor may considerthe contention resolution procedure according to transmission of thethird message as a successful procedure when the PDCCH including the ULgrant information is received through the first DL CC and indicatestemporary C-RNTI of the UE, and a PDSCH corresponding to the PDCCHincludes the UE contention resolution identity MAC control element.

The processor may consider whether the PDCCH is received through thefirst DL CC or not in order to determine whether the contentionresolution procedure according to transmission of the third message issuccessful only when the third message including the C-RNTI MAC controlelement is transmitted to the eNB.

The UE may perform a HARQ operation through a second UL CC differentfrom the first UL CC and a second DL CC different from the first DL CC,independently of transmission of the third message and reception of theUL grant information.

Advantageous Effects

In accordance with embodiments of the present invention as describedabove, it is possible to prevent a contention resolution procedure frombeing erroneously ended due to wrong analysis of an uplink grant signal.

DESCRIPTION OF DRAWINGS

FIG. 1 is a view referred to for describing the configuration of a LongTerm Evolution (LTE) system;

FIGS. 2 and 3 illustrate radio protocol layers;

FIG. 4 is a view referred to for describing Carrier Aggregation (CA);

FIG. 5 is a diagram illustrating a signal flow for operations of a UserEquipment (UE) and an evolved Node B (eNB) in a contention-based randomaccess procedure;

FIG. 6 is a view referred to for describing an example of confusion thata UE may experience during a contention resolution procedure whenperforming contention-based random access to an eNB;

FIG. 7 is a view referred to for describing an example of generation ofwrong contention resolution in a contention-based random accessprocedure in the LTE system;

FIG. 8 is a view referred to for describing a method for performingrandom access to an eNB by a UE according to an embodiment of thepresent invention; and

FIGS. 9 and 10 illustrate the structures of processors at the UE and eNBaccording to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that canbe implemented according to the invention. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details. For example, the following detaileddescription is given under the assumption that a system conforming toone of the 3GPP LTE series of standards is being used. However, thedescription is applicable to any other mobile communication systemexcept for specific features inherent to the 3GPP LTE series ofstandards.

In some instances, known structures and devices are omitted or are shownin block diagram form, focusing on important features of the structuresand devices, so as not to obscure the concept of the invention. The samereference numbers will be used throughout this specification to refer tothe same or like parts.

In the following description, the term terminal generically refers to amobile or fixed user terminal device such as a User Equipment (UE), aMobile Station (MS), etc. In addition, the term Base Station (BS)generically refers to any node at a network end which communicates witha UE, such as a Node B, an eNode B, etc.

A description will be given of a method for performing a random accessprocedure by a UE for an eNB without confusion in a contentionresolution procedure in a mobile communication system to which CA isapplied and a UE configured to perform the method. To achieve this, acontention resolution procedure and a scheme for preventing theabove-mentioned confusion will be described in more detail.

Contention is generated during a random access procedure because thenumber of random access preambles is finite. Since an eNB cannot providerandom access preambles to respective UEs, a UE selects one of commonrandom access preambles and transmits the selected random accesspreamble. In this case, while two or more UEs may select the same randomaccess preamble and transmit it through the same PRACH resource, the eNBdetermines that one random access preamble is transmitted from one UE,and thus transmits a random access response to the one UE and predictsthat the one UE will receive the random access response. However, thetwo or more UEs receive the random access response when contentionoccurs as described above, and thus they perform operations depending onthe random access response. That is, the two or more UEs transmitdifferent data items through the same radio resource using one uplinkgrant included in the random access response.

Accordingly, all the data items may fail in being transmitted to theeNB, or the eNB may receive only data from a specific UE depending onthe location or transmit power of the UE. In the latter case, since thetwo or more UEs consider that they have succeeded in transmitting theirdata items, the eNB needs to inform UEs which fail in the contentionthat they do not succeed in transmitting their data items. Informing UEsof information about success or failure of contention is calledcontention resolution.

There are two contention resolution methods, a method using a contentionresolution (hereinafter referred to as CR) timer and a method oftransmitting an identifier of a successful UE to other UEs. The formercase is used when a UE previously has its C-RNTI before a random accessprocedure. Specifically, the UE having its C-RNTI transmits dataincluding the C-RNTI to the eNB in response to a random access responseand activates a CR timer. Upon reception of PDCCH information includingthe C-RNTI before the expiration of the CR timer, the UE determines thatit has succeeded in the contention and normally ends the random accessprocedure. On the contrary, when the UE does not receive the PDCCHinformation including its C-RNTI before the expiration of the CR timer,the UE determines that it has failed in the contention and re-executesthe random access procedure or informs a higher layer of the failure.

The latter case, that is, the method of transmitting an identifier of asuccessful UE, is used when the UE has no C-RNTI before a random accessprocedure. Specifically, when the UE has no C-RNTI, the UE includes anidentifier higher than the C-RNTI (S-TMSI or random access ID;hereinafter referred to as a UE contention resolution identify MACcontrol element) in data depending on uplink grant information includedin a random access response, transmits the data, and activates a CRtimer. Upon reception of data including the S-TMSI through a DL-SCHbefore the expiration of the CR timer, the UE may determine that therandom access procedure is successful. On the contrary, when the UE doesnot receive the data including the S-TMSI through the DL-SCH, the UEdetermines that the random access procedure has failed.

Confusion that a UE may experience will be described on the basis of theabove-described concept of contention resolution.

FIG. 6 is a view referred to for describing an example of confusion thata UE may experience during a contention resolution procedure whenperforming contention-based random access to an eNB.

The UE may transmit a Buffer Status Report (hereinafter referred to asBSR(A)) to the eNB according to generation of new data in step 1. Here,it is assumed that the UE has an uplink radio resource for transmittingthe BSR(A).

In step 2, if data having priority higher than current data stored in abuffer is generated in the UE, the UE needs to transmit another BSR(B)to the eNB in order to report generation of the data to the eNB. At thistime, it is assumed that the UE does not have an uplink radio resourcefor transmitting the BSR(B), that is, the UE performs a contention-basedrandom access procedure to request the eNB to provide the uplink radioresource.

Meanwhile, the eNB cannot determine whether the UE currently performsthe random access procedure or not before correctly receiving a thirdmessage during the random access procedure. In the example of FIG. 6, ifthe eNB transmits an uplink grant for the previously received BSR(A) tothe UE in a state that the eNB has not correctly received the thirdmessage transmitted from the UE, the UE may determine the uplink grantto be a contention resolution message for the random access procedure totransmit the BSR(B) and determine that the random access procedure issuccessful. Accordingly, the BSR(B) that the UE attempts to transmit maybe lost.

Although the LTE system has the problem that a BSR is lost duringtransmission due to wrong contention resolution illustrated in FIG. 6,this problem is not significant because the frequency of generation islow. In the LTE-A system, however, the frequency of generation oftransmission loss of BSR due to wrong contention resolution in acontention-based random access procedure may increase since a UE uses aplurality of UL CCs and DL CCs.

FIG. 7 is a view referred to for describing an example of generation ofwrong contention resolution in a contention-based random accessprocedure in the LTE system.

In FIG. 7, it is assumed that a UE performs communication using two DLCCs DL(A) and DL(B) and an eNB carries out communication using two ULCCs UL(A) and UL(B), in which UL(A) and DL(A) are connected to eachother and UL(B) and DL(B) are connected to each other. The UE canacquire information about available CCs and information about UL CC/DLCC connection through system information received from the eNB.

Referring to FIG. 7, the UE may transmit BSR(A) to the eNB through UL(A)according to generation of new data in step S701. Here, it is assumedthat the UE has an uplink radio resource for transmitting the BSR(A).When the eNB transmits a NACK message in response to the BSR(A)transmitted in step S701, the UE may retransmit the BSR(A) (S703).

If data having priority higher than current data stored in a buffer isgenerated in the UE, the UE needs to transmit another BSR BSR(B) to theeNB in order to report generation of the data to the eNB. However, ifthe UE does not have an uplink radio resource for transmitting theBSR(B), that is, if the UE performs a contention-based random accessprocedure through UL(A) to request the eNB to provide the uplink radioresource, the UE may transmit a first message through UL(A) (S704), andthe eNB may transmit a second message to the UE through DL(A) inresponse to the first message. The UE may transmit a third messageincluding its identifier to the eNB through UL(A) using uplink grantinformation of the received second message (S706).

The eNB cannot determine whether the UE currently performs the randomaccess procedure or not before correctly receiving the third messageduring the random access procedure. In the example of FIG. 7, if the eNBtransmits an uplink grant for the previously received BSR(A) to the UEthrough DL CC(B) in a state that the eNB has not correctly received thethird message transmitted from the eNB (S707), the UE may determine theuplink grant received in step S707 as a contention resolution messagefor the random access procedure to transmit the BSR(B) and determinethat the random access procedure is successful. Accordingly, the BSR(B)that the UE attempts to transmit may be lost.

To prevent confusion of a UE in a system using the CA technology, anembodiment of the present invention determines that contentionresolution of a random access procedure is successful only in the casein which the UE receives a contention resolution control signal througha specific DL CC. That is, when the UE does not receive uplink grantinformation through a DL CC corresponding to an UL CC through which thethird message is transmitted, the UE according to the embodiment of thepresent invention does not consider the corresponding uplink grantsignal as an uplink grant signal for a contention resolution procedure.

FIG. 8 is a view referred to for describing a method for performing arandom access procedure by a UE for an eNB according to an embodiment ofthe present invention.

In the example of FIG. 8, it is assumed that the UE is configured to beable to use two UL CCs UL(A) and UL(B) and two DL CCs DL(A) and DL(B)from the eNB for CA, UL(A) and DL(A) are mapped to each other and UL(B)and the DL(B) are mapped to each other, and UL(A) and UL(B) are assignedPRACH resources such that they can be used to perform a random accessprocedure.

The UE may transmit a random access preamble to the eNB through UL(A) inorder to perform a contention-based random access procedure (S801). TheeNB may transmit a random access response to the UE through DL(A) mappedto UL(A) in response to the random access preamble (S802). The UE maytransmit a third message through UL(A) using uplink grant informationincluded in the random access response (S803). The third messageincludes the identifier of the UE. The identifier of the UE may beC-RNTI. The identifier of the UE is assumed to be xyz in the embodimentof the present invention.

The UE may receive a contention resolution message including theidentifier of the UE, xyz, from the eNB through DL(B) (S804).Specifically, the contention resolution message may include uplink grantinformation and may be a PDCCH which indicates xyz of the UE. However,it is assumed that the UE according to the embodiment of the presentinvention considers the PDCCH including the uplink grant information asthe contention resolution message only when the PDCCH is receivedthrough DL(A) corresponding to UL(A) through which the random accesspreamble is transmitted or the third message. Accordingly, the UE maynot determine the uplink grant information transmitted through DL(B) asthe contention resolution message (S805). In this case, the UEcontinuously attempts to receive the contention resolution message untilits CR timer expires.

The UE may receive an uplink grant signal including the identifier ofthe UE, xyz, through DL(A) from the eNB (S806). Here, it is assumed thatthe CR timer of the UE has not expired yet. Accordingly, the UE mayconsider the uplink grant signal as a contention resolution messagesince the UE determines that the uplink grant signal has been receivedthrough DL(A) mapped to UL(A) through which the random access preambleor the third message is transmitted, and determine whether thecorresponding identifier is matched (S808). In the embodiment of thepresent invention, the UE receives the uplink grant information throughDL(A) corresponding to UL(A) used to transmit the random access preambleand the third message, and may determine that the random accessprocedure is successful since it has received the PDCCH which indicatesthe identifier of the UE, xyz.

As described above with reference to FIG. 8, a received DL CC may bechecked to determine whether a received uplink grant signal can be usedas a contention resolution message or not only when the UE has itsC-RNTI before a random access procedure. When the UE attempts contentionresolution using a UE contention resolution identity control elementbecause it does not include its C-RNTI before initiation of the randomaccess procedure, the UE needs to receive a PDCCH which indicatestemporary C-RNTI and determine whether the PDSCH corresponding to thePDCCH includes the UE contention resolution identity control element ornot. Accordingly, the probability that the UE confuses uplink grantinformation regarding a corresponding BSR with uplink grant informationregarding another BSR, as described with reference to FIG. 7 is verylow. The present invention may be configured to consider a DL CC throughwhich a fourth message is transmitted for all contention-based randomaccess procedures.

A description will be given of a detailed configuration for allowing theUE to consider only uplink grant information received through a DL CCcorresponding to an UL CC used to transmit a first message/third messageas a contention resolution message as described above with reference toFIG. 8.

In general, a contention resolution algorithm in a random accessprocedure performed in the MAC layer of the UE is as follows.

TABLE 1 In the case in which a lower layer (that is, physical layer)reports reception of a PDCCH to the UE, (1) when a third messageincludes a C-RNTI MAC control element (that is, when the UE includes itsC-RNTI before initiation of the random access procedure), (A) if therandom access procedure is a contention-based random access procedure(that is, if the random access procedure is initiated in a MACsublayer), and the PDCCH indicates the C-RNTI and includes uplink grantinformation for new transmission, or (B) if the random access procedureis a contention free random access procedure (that is, if the randomaccess procedure is initiated by a PDCCH command) and the received PDCCHindicates C-RNTI, (a) the UE determines that the contention resolutionprocedure is successful, and (b) stops its CR timer. (2) In the case inwhich the third message includes CCCH SDU, the received PDCCH indicatestemporary C-RNTI of the UE, and a received MAC PDU has been successfuldecoded, (A) the UE stops its CR timer, and (B) when the correspondingMAC PDU includes a UE contention resolution identity control element,and this contention resolution identity control element is matched tothe CCCH SDU transmitted through the third message (that is, thecontention resolution identity control element corresponds to theidentity of the UE, transmitted from the UE), (a) the UE considered thecontention resolution procedure to be successful, and (b) sets thetemporary C_RNTI used for the random access as C-RNTI.

In the algorithm as shown in Table 1, the UE may consider a DL CCthrough which the UE receives an uplink grant signal as follows.

TABLE 2 In the case in which a lower layer (that is, physical layer)reports reception of a PDCCH through a DL CC corresponding to an UL CCused to transmit a random access preamble or a third message to the UE,(1) when the third message includes a C-RNTI MAC control element (thatis, when the UE includes its C-RNTI before initiation of the randomaccess procedure), (A) if the random access procedure is acontention-based random access procedure (that is, if the random accessprocedure is initiated in a MAC sublayer), and the PDCCH indicates theC-RNTI and includes uplink grant information for new transmission, or(B) if the random access procedure is a contention free random accessprocedure (that is, if the random access procedure is initiated by aPDCCH command) and the received PDCCH indicates C-RNTI, (a) the UEdetermines that the contention resolution procedure is successful, and(b) stops its CR timer. (2) In the case in which the third messageincludes CCCH SDU, the received PDCCH indicates temporary C-RNTI of theUE, and a received MAC PDU has been successful decoded, (A) the UE stopsits CR timer, and (B) when the corresponding MAC PDU includes a UEcontention resolution identity control element, and this UE contentionresolution identity control element is matched to the CCCH SDUtransmitted through the third message (that is, the UE contentionresolution identity control element corresponds to the identity of theUE, transmitted from the UE), (a) the UE considers the contentionresolution procedure to be successful, and (b) sets the temporary C_RNTIused for the random access as C-RNTI.

Meanwhile, a description will be given of a scheme of considering a DLCC through which the UE receives an uplink grant signal only in arestricted case in which the UE has its C-RNTI before initiation of arandom access procedure according to an embodiment of the presentinvention and the random access procedure is initiated by the UE,specifically, a MAC sublayer of the UE.

TABLE 3 In the case in which a lower layer (that is, physical layer)reports reception of a PDCCH to the UE, (1) when the third messageincludes a C-RNTI MAC control element (that is, when the UE includes itsC-RNTI before initiation of the random access procedure), (A) if therandom access procedure is a contention-based random access procedure(that is, if the random access procedure is initiated in the MACsub-layer), the PDCCH is received through a DL CC corresponding to an ULCC used to transmit a random access preamble or the third message, andthe received PDCCH indicates the C-RNTI and includes uplink grantinformation for new data transmission, or (B) if the random accessprocedure is a contention free random access procedure (that is, if therandom access procedure is initiated by a PDCCH command) and thereceived PDCCH indicates C-RNTI, (a) the UE determines that thecontention resolution procedure is successful, and (b) stops its CRtimer. (2) In the case in which the third message includes CCCH SDU, thereceived PDCCH indicates temporary C-RNTI of the UE, and a received MACPDU has been successful decoded, (A) the UE stops its CR timer, and (B)when the corresponding MAC PDU includes a UE contention resolutionidentity control element, and this UE contention resolution identitycontrol element is matched to the CCCH SDU transmitted through the thirdmessage (that is, the UE contention resolution identity control elementcorresponds to the identity of the UE, transmitted from the UE), (a) theUE considers the contention resolution procedure to be successful, and(b) sets the temporary C_RNTI used for the random access as C-RNTI.

Meanwhile, a description will be given of a scheme of considering a DLCC through which the UE receives an uplink grant signal only in arestricted case in which the UE has its C-RNTI before initiation of arandom access procedure according to an embodiment of the presentinvention and the random access procedure is initiated by a command ofthe eNB, specifically, a PDCCH command.

TABLE 4 In the case in which a lower layer (that is, physical layer)reports reception of a PDCCH to the UE, (1) when the third messageincludes a C-RNTI MAC control element (that is, when the UE includes itsC-RNTI before initiation of the random access procedure), (A) if therandom access procedure is a contention-based random access procedure(that is, if the random access procedure is initiated in a MACsublayer), and the PDCCH indicates the C-RNTI and includes uplink grantinformation for new data transmission, or (B) if the random accessprocedure is a contention free random access procedure (that is, if therandom access procedure is initiated by a PDCCH command), the PDCCH isreceived through a DL CC corresponding to an UL CC used to transmit arandom access preamble or a third message, and the received PDCCHindicates the C-RNTI, (a) the UE determines that the contentionresolution procedure is successful, and (b) stops its CR timer. (2) Inthe case in which the third message includes CCCH SDU, the receivedPDCCH indicates temporary C-RNTI of the UE, and a received MAC PDU hasbeen successful decoded, (A) the UE stops its CR timer, and (B) when thecorresponding MAC PDU includes a UE contention resolution identitycontrol element, and this UE contention resolution identity controlelement is matched to the CCCH SDU transmitted through the third message(that is, the UE contention resolution identity control elementcorresponds to the identity of the UE, transmitted from the UE), (a) theUE considers the contention resolution procedure to be successful, and(b) sets the temporary C_RNTI used for the random access as C-RNTI.

Meanwhile, the following algorithm may be used when the UE considers aDL CC through which an uplink grant signal is received only which the UEdoes not have its C-RNTI before initiation of a random access procedure.

TABLE 5 In the case in which a lower layer (that is, physical layer)reports reception of a PDCCH to the UE, (1) when a third messageincludes a C-RNTI MAC control element (that is, when the UE includes itsC-RNTI before initiation of the random access procedure), (A) if therandom access procedure is a contention-based random access procedure(that is, if the random access procedure is initiated in a MACsublayer), and the PDCCH indicates the C-RNTI and includes uplink grantinformation for new data transmission, or (B) if the random accessprocedure is a contention free random access procedure (that is, if therandom access procedure is initiated by a PDCCH command) and thereceived PDCCH indicates C-RNTI, (a) the UE determines that thecontention resolution procedure is successful, and (b) stops its CRtimer. (2) In the case in which the third message includes CCCH SDU, thereceived PDCCH indicates temporary C-RNTI of the UE, and a received MACPDU has been successful decoded, (A) the UE stops its CR timer, and (B)when the PDCCH is received through a DL CC corresponding to an UL CCused to transmit a random access preamble or the third message, thecorresponding MAC PDU includes a UE contention resolution identitycontrol element, and this UE contention resolution identity controlelement is matched to the CCCH SDU transmitted through the third message(that is, the UE contention resolution identity control elementcorresponds to the identity of the UE, transmitted from the UE), (a) theUE considers the contention resolution procedure to be successful, and(b) sets the temporary C_RNTI used for the random access as C-RNTI.

The algorithms shown in Tables 2, 3, 4 and 5 may be used in a combinedmanner.

Now, a description will be given of a UE apparatus and an eNB apparatusfor performing random access to the eNB as described above.

The UE apparatus and the eNB apparatus include an antenna, a processor,etc. according to device type. The following description is given,focusing on the structure of a processor for controlling theabove-described operations.

The processors of the UE and the eNB may have the layer structuresillustrated in FIGS. 2 and 3. In each embodiment of the presentinvention, the processors of the UE and the eNB have the followingstructures for use in a system to which CA is applied.

FIGS. 9 and 10 illustrate the structures of processors at the UE and theeNB according to an embodiment of the present invention.

Specifically, FIG. 9 illustrates the downlink L2 structure of the eNBfor performing the afore-described method, and FIG. 10 illustrates theuplink L2 structure of the UE for performing the afore-described method.

The CA technology significantly affects the MAC layer at L2. Forexample, a system using CA uses a plurality of CCs and each HARQ entitymanages one CC. Therefore, the MAC layers of the UE processor and theeNB processor should perform operations related to a plurality of HARQentities in the embodiment of the present invention. In addition, sinceeach HARQ entity processes a transport block, a plurality of transportblocks can be transmitted or received at the same time on a plurality ofCCs in CA.

That is, MAC layer modules of the UE and the eNB include a plurality ofHARQ entities, each taking charge of a CC in the embodiment of thepresent invention. As illustrated in FIG. 8, the MAC layer module of theeNB processor may include a multiplexing module for multiplexing aplurality of HARQ entities corresponding 1:1 to a plurality of CCs foreach UE (e.g. UE1 or UE2) and a module for scheduling/priority handlingfor all UEs. As illustrated in FIG. 10, the MAC layer module of the UEmay also include a plurality of HARQ entities respectively correspondingto a plurality of CCs, and may include a module for scheduling/priorityhandling for uplink resources.

The UE apparatus having the above L2 structure will be described ingreater detail, from the viewpoint of the afore-described random accessprocedure.

The UE processor includes an RRC layer module for generating a logicalchannel signal for RRC connection establishment or reestablishment. Forexample, an RRC connection request message may be mapped to a CCCH andtransmitted to the later-described MAC layer.

The MAC layer module may map the logical channel signal (e.g. the CCCH)for RRC connection establishment/reestablishment to a MAC PDU format andtransmit the mapped logical channel signal to the physical layer over atransport channel. The transport channel may be transmitted using any ofa plurality of HARQ entities. A physical layer module may map thetransport channel signal to a physical channel (e.g. a PDSCH) andtransmit the PDSCH to the eNB. In case of the above-described signal forRRC connection establishment/reestablishment, it may be transmitted tothe eNB as a third message transmitted in a random access procedure.Meanwhile, first and second messages may be transmitted according todecision of the physical layer irrespective of HARQ entities in therandom access procedure.

Meanwhile, the UE processor according to the embodiment of the presentinvention may be configured such that it does not consider an uplinkgrant signal received during a contention resolution procedure as acontention resolution message when the uplink grant signal is notreceived through a DL CC corresponding to a UL CC used to transmit arandom access preamble or a third message. Specifically, a MAC layer ofa UE processor according to an embodiment of the present invention maybe configured to execute the contention resolution algorithms as shownin Tables 2, 3, 4 and 5. The algorithms of Tables 2, 3, 4 and 5 may beimplemented in a MAC layer module in a software or hardware manner.

The detailed description of the preferred embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific embodiments described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

While the above-described embodiments have been described, focusing onthe 3GPP LTE system, the present invention is not limited thereto andcan be used for various mobile communication systems using a combinationof a plurality of component carriers.

The invention claimed is:
 1. A method for performing a random accessprocedure in a mobile communication system supporting carrieraggregation using a primary carrier and one or more secondary carriers,the method performed by a User Equipment (UE) and comprising:transmitting a first message comprising a random access preamble to anevolved NodeB (eNB) through the primary carrier; receiving a secondmessage including first uplink (UL) grant information as a response tothe random access preamble through the primary carrier; transmitting athird message including identity information of the UE to the eNBthrough an uplink radio resource corresponding to the first UL grantinformation; and receiving a Physical Downlink Control Channel (PDCCH)indicated by the identity information of the UE from the eNB, wherein,when the PDCCH is received through the primary carrier, the UE considersa contention resolution procedure according to transmission of the thirdmessage as a successful procedure, and wherein, when the PDCCH isreceived through the one or more secondary carriers, the UE considersthe contention resolution procedure as an unsuccessful procedure.
 2. Themethod according to claim 1, wherein the UE includes a physical layermodule and a Medium Access Control (MAC) layer module, and wherein theMAC layer module determines that the contention resolution procedure issuccessful only when the physical layer module reports reception of thePDCCH through the primary carrier to the MAC layer module.
 3. The methodaccording to claim 1, wherein the identity information of the UE is aCell-Radio Network Temporary Identifier (C-RNTI) Medium Access Control(MAC) control element which indicates C-RNTI of the UE, the randomaccess procedure is initiated by the UE, and wherein, when the PDCCH isreceived through the primary carrier, indicates the C-RNTI of the UE,and includes second uplink grant information for new data transmission,the contention resolution procedure is considered as a successfulprocedure.
 4. The method according to claim 3, wherein the UE considerswhether the PDCCH is received through the primary carrier in order todetermine whether the contention resolution procedure is successful onlywhen the UE transmits the third message including the C-RNTI MAC controlelement to the eNB.
 5. The method according to claim 1, wherein theidentity information of the UE is a Cell-Radio Network TemporaryIdentifier (C-RNTI) Medium Access Control (MAC) control element whichindicates C-RNTI of the UE, the random access procedure is initiated bya command of the eNB, and wherein, when the PDCCH is received throughthe primary carrier and indicates the C-RNTI of the UE, the contentionresolution procedure is considered as a successful procedure.
 6. Themethod according to claim 5, wherein when the PDCCH is received throughthe primary carrier and indicates the C-RNTI of the UE, the contentionresolution procedure is considered as a successful procedure in a casein which the PDCCH includes second UL grant information and in a case inwhich the PDCCH includes downlink assignment information.
 7. The methodaccording to claim 1, wherein the identity information of the UE is a UEcontention resolution identity Medium Access Control (MAC) controlelement which includes an identifier other than a Cell-Radio NetworkTemporary Identifier (C-RNTI) of the UE, and wherein, when the PDCCH isreceived through the primary carrier and indicates temporary C-RNTI ofthe UE, and a Physical Downlink Shared Channel (PDSCH) corresponding tothe PDCCH includes the UE contention resolution identity MAC controlelement, a contention resolution procedure is considered as a successfulprocedure.
 8. The method according to claim 1, wherein the UE performs aHARQ operation through the one or more secondary carriers, independentlyof transmission of the third message and reception of the PDCCH.
 9. Themethod according to claim 1, wherein the primary carrier is a standalonecarrier that is usable without the one or more secondary carriers, andeach of the one or more secondary carriers is an additional carrier thatis not usable without the primary carrier.
 10. A UE which performsrandom access to an evolved NodeB (eNB) in a mobile communication systemsupporting carrier aggregation using a primary carrier and one or moresecondary carriers, the UE comprising: a Medium Access Control (MAC)layer module including a plurality of HARQ entities respectivelycorresponding to the plurality of component carriers, and configured tocontrol signal transmission and reception using the primary carrier orthe one or more secondary carriers; and a processor functionallyconnected to the MAC layer module and including a physical layer moduleconfigured to transmit and receive a signal through the primary carrieror the one or more secondary carriers, wherein only when a PhysicalDownlink Control Channel (PDCCH) indicated by identity information ofthe UE is received through the primary carrier used to transmit amessage to the eNB, the processor considers a contention resolutionprocedure according to transmission of the message as a successfulprocedure, and wherein when the PDCCH is received through the one ormore secondary carriers, the processor considers the contentionresolution procedure as an unsuccessful procedure.
 11. The UE accordingto claim 10, wherein the MAC layer module is further configured todetermine whether the contention resolution procedure is successful onlywhen the physical layer module reports reception of the PDCCH throughthe primary carrier to the MAC layer module.
 12. The UE according toclaim 10, wherein the message is identity information of the UE andincludes a Cell-Radio Network Temporary Identifier (C-RNTI) MediumAccess Control (MAC) control element which indicates C-RNTI of the UE,and the processor considers the contention resolution procedure as asuccessful procedure when the random access procedure is initiated bythe UE, the PDCCH is received through the primary carrier, indicates theC-RNTI of the UE, and includes uplink grant information for new datatransmission.
 13. The UE according to claim 12, wherein the processorconsiders whether the PDCCH is received through the primary carrier ornot in order to determine whether the contention resolution procedure issuccessful only when the third message including the C-RNTI MAC controlelement is transmitted to the eNB.
 14. The UE according to claim 10,wherein the message is identity information of the UE and includes aCell-Radio Network Temporary Identifier (C-RNTI) Medium Access Control(MAC) control element which indicates C-RNTI of the UE, and theprocessor considers the contention resolution procedure as a successfulprocedure when the random access procedure is initiated by a command ofthe eNB, and the PDCCH is received through the primary carrier andindicates the C-RNTI of the UE.
 15. The UE according to claim 14,wherein when the PDCCH is received through the primary carrier andindicates the C-RNTI of the UE, the processor considers the contentionresolution procedure as a successful procedure in a case in which thePDCCH includes uplink grant information and in a case in which the PDCCHincludes downlink assignment information.
 16. The UE according to claim10, wherein the message is identity information of the UE and includes aUE contention resolution identity MAC control element including anidentifier other than a Cell-Radio Network Temporary Identifier (C-RNTI)of the UE, and the processor considers the contention resolutionprocedure as a successful procedure when the PDCCH is received throughthe primary carrier and indicates temporary C-RNTI of the UE, and aPhysical Downlink Shared Channel (PDSCH) corresponding to the PDCCHincludes the UE contention resolution identity MAC control element. 17.The UE according to claim 10, wherein the UE performs a HARQ operationthrough the one or more secondary carriers, independently oftransmission of the message and reception of the PDCCH.
 18. The UEaccording to claim 10, wherein the primary carrier is a standalonecarrier that is usable without the one or more secondary carriers, andeach of the one or more secondary carriers is an additional carrier thatis not usable without the primary carrier.